1. Field of the Invention
The present invention relates to an achromatic lens system, and more precisely relates to a color image reading lens system for office supplies.
2. Description of the Related Art
There is known an achromatic lens system comprised of a special low-dispersion glass which is however expensive, easily cracked, and easily influenced by a change in the ambient temperature.
There is also known an achromatic lens system using no low-dispersion glass, as disclosed for example in Japanese Un-examined Patent Publication Nos. 3-96912 or 5-142468 in which a chromatic aberration correcting lens is provided within the lens system. However, the optical arrangement disclosed in JPP '912 in which a cemented lens and a plane-parallel plate are interchanged to achromatize three wavelengths is complicated. In the optical arrangement disclosed in JPP '468, the range of wavelength (wavelength band) to be achromatized is relatively narrow, i.e., within 486 nm (F-ray) to 656 nm (C-ray).